This invention is related to airfoils, and more particularly to composite airfoils.
Gas turbine engines comprise one or more rotating turbines that are used to extract energy from a high velocity and high temperature gas flow produced within the gas turbine engine. The turbines are comprised of a plurality of radially extending airfoil blades that are connected at their inner diameter ends to a rotor, which is connected to a shaft that rotates within the engine as the blades interact with the gas flow. The rotor typically comprises a disk having a plurality of axial retention slots that receive mating root portions of the blades to prevent radial dislodgment. Blades typically also include integral inner diameter platforms that prevent the high temperature gases from escaping through the radial retention slots. Between stages of the turbine blades are disposed a plurality of radially extending stationary airfoil vanes, which are typically supported by inner and outer diameter shrouds that are suspended from an outer engine case and supported by an inner structure, respectively. During operation of the engine, the turbine blades and vanes are subjected to high heat from combustion products. Additionally, the blades are subjected to large centrifugal loads due to spinning and large gas loads causing twisting and bending of the airfoil. As a result, the outermost surface of the airfoil is the most highly stressed. It is, therefore, a constant design challenge to develop materials for turbine blades and vanes that are more heat resistant to reduce cooling demands, lighter to increase propulsive efficiencies in aircraft engines and stronger to increase stress resistance and capability of the blade.
Typically, turbine blades and vanes are fabricated from high strength alloys as single pieces, with integral roots, platforms and shrouds. More recent blade designs have attempted to incorporate ceramic matrix composite (CMC) materials or organic matrix composite (OMC) materials, which are lightweight, heat resistant and strong. CMC material comprises a ceramic fabric that is infused with a pre-ceramic matrix. The ceramic fabric is preformed to a desired shape and the pre-ceramic matrix is converted into a ceramic matrix to produce a part having the lightweight and heat resistance characteristics of the matrix and the strength characteristics of the fabric. OMC material comprises a woven fabric or unidirectional collection of fibers infused with a resin. The fibers may be carbon, glass, aramide or a combination of fiber types. The fabric is preformed to a desired shape, and the resin is cured to form a rigid, lightweight component.
It is desirable to use CMC or OMC material, as the materials can weigh approximately one third of the weight of typical metal alloys used for components, while providing strength and having much higher temperature limitations. However, CMC and OMC blades are difficult to design due to high tensile stress in the leading edge and trailing edge regions. This high stress is a result of the combined radial pull load and gas load bending on the blade during engine operation. The trailing edge is particularly critical due to its requirement to be very thin.